The present invention relates to determining the level of a liquid or semi-liquid, such as a slurry, in a container, utilizing elements that can be supported on the exterior of the container so that they are isolated from the liquid being measured.
In sealed liquid containers, the determination of the liquid level is important for various operations. It is desired to measured the liquid level without having components that are mounted on the interior of the container, and yet provide accurate indications of the liquid level on the interior. Additionally, when such containers are used in aircraft for various applications, it is necessary to minimize the amount of power consumed, as well as insuring the reliability in many different operating conditions. Level sensing arrangements can be used to indicate one or two desired levels or to provide a substantially continuous level indication.
The present invention relates to a non-intrusive level sensor assembly for determining the level of a liquid in a container, and a method of carrying out the measurement.
Sealed containers are used for carrying many different liquids, some of which can be highly corrosive, or otherwise hazardous. The ability to measure the level of a liquid is important, and it is desired to provide for the measurement without substantial effects from acceleration.
The present invention uses a heat source, and at least two temperature sensitive elements (also called temperature sensors) mounted on a wall of the tank, preferably on the exterior surface. One temperature sensitive element is adjacent the heat source and the other temperature sensitive element is spaced from the heat source. The temperature sensor adjacent the heat source measures the temperature rise due to the power applied to the heated element.
The heat source adds heat to the tank wall and raises the temperature of the tank wall in the localized area. This temperature is measured by the one adjacent temperature sensitive element. The second one of the temperature sensitive elements or sensors is positioned on the tank wall at a location spaced from the heat source sufficiently so that the temperature in the area of the second temperature sensitive element is not substantially affected by the heat from the heat source. The second sensor is horizontally spaced, at the same vertical level as the heated sensor.
The differential in the temperatures sensed by the two temperature sensitive elements or sensors, changes when the inner surface of the tank wall aligned with the sensors is in contact with a liquid. Liquid conducts heat from the wall so the temperature differential is different when the liquid level is below the sensors and the inner surface of the wall is in contact with a gas or air. The difference in the sensed temperatures can be used as an indication of whether or not the liquid is at or above the level of the temperature sensitive elements. In the form shown, the heat source is driven to generate more heat when there is a liquid at the level of the heated temperature sensitive element to maintain the temperature differential the same as when the liquid level drops below the measuring plane.
When there is liquid on an opposite side of the tank wall from the temperature sensitive element, the heat source will require more power to provide additional heat to maintain the same temperature differential as without liquid at that level, and this change in power requirement is calibrated to indicate that the liquid level is at the temperature sensor pair.
Alternatively, if desired, the changes in the temperature differential can be used for directly indicating whether or not the surface of the wall at the level of the temperature sensitive elements or sensor is in contact with the liquid.
The level sensor of the present invention will operate satisfactorily with tanks made of substantially any material, although the difference in the heat transfer to a liquid or a gas is more pronounced where the tank material is highly heat conductive. The temperature sensitive elements or sensors have to be thermally bonded to the wall, as does the heater, in order to have an efficient operation. The tank shown is oriented horizontally. In some instances, the tank can be upright. The liquid level also can be measured in a tank that is standing vertically.